1. Field of the Invention
The present invention relates in general to remotely programmable automatic telephone call forwarding and switching devices, commonly referred to as call diverters, with variable memories and having improved ring detection, dial tone detection, automatic line bridging, an improved audio bridge and remote programming capabilities. This automatic diverter incorporates circuits which function to answer telephone calls, bridge telephone lines, dial preprogrammed numbers, reprogram its memory locally or remotely, and insure compatibility with the wide diversity of telephone equipment and operational standards now in use including compatability with rotary and touch dial systems.
2. Description of the Prior Art
Call diverters of varying capability have been commercially available for some time. They have had very minimal consumer acceptance, however, for a varity of reasons. The prime reason for their lack of acceptance is their failure to operate reliably with existing telephone equipment and operational conditions, which vary widely from exchange to exchange, from system to system and from company to company. Each individual telephone call diverter marketed in the past has suffered from one or more major deficiencies which effectively inhibited its value and utility. In general, the more common deficiencies prevalent in the prior art are timing problems encountered in handling calls between exchanges, timing problems in the occurrence of dial tones, failure to recognize the wide range of dial tones provided by various exchanges and systems, the confusion of noise on telephone lines with dial tones, and incompatability with both rotary dial and touch dial equipment. A host of minor problems are also present in the prior art units. Furthermore, the prior art units were very expensive to manufacture.
The present invention represents an improvement over the prior art based on careful analysis of and experimentation with existing units, incorporating many minor improvements in operation and circuitry and major improvements in timing, memory, dial tone detection, line bridging and compatability with existing telephone equipment.
Many of the improvements herein disclosed are directed both to improve operational performance and to savings in costs of manufacturing, installation and maintenance.
The prior art is best represented in U.S. Pat. No. 3,704,346 which presents a detailed analysis of the nature, functions and circuitry of a remotely programmable call diverter and fairly represents the state of the art as of its filing date. This disclosure will not, so far as possible, repeat those functional explanations common to all call diverters which appear in that prior patent, but will stress the differences and improvements of the present diverter.
Any remotely programmable call forwarding device of necessity requires that the telephone subscriber have two separate telephone lines, one for receiving incoming calls, henceforth called line A, and another, henceforth called line B, for carrying the forwarded call from the call diverting device to the telephone instrument at the number the call has been diverted to. In the present invention, line B is also used to reprogram by touch dial techniques a new telephone number into the diverter's memory from a location remote from the diverter itself. Such call forwarding devices must also include ring detection means, dial tone detection means, means to store a telephone number in memory, means to bridge lines A and B, means to dial out the telephone number stored in memory, and means to program or reprogram the device from a remote location. In contrast to prior art devices, it is also desirable that there be some way to verify from a remote location that a newly programmed telephone number has been properly stored in the diverting device's memory, as in the present disclosure.
The improved dial tone detector in the present invention enables the diverter disclosed in this application to overcome the significant problem in the prior art which arises because other dial tone detectors are frequently deceived into interpreting as a dial tone some other signal or noise commonly found on telephone lines. For example, some prior art dial tone detectors commonly monitor the telephone lines for the persistence over some appropriate time period of a particular amplitude level in the audio frequency range. Other prior art dial tone detectors look for a particular frequency or frequencies of dial tones. But the audio frequency range is very broad (from approximately 300 to 3000 Hz) and signals other than dial tone signals may and often do persist on telephone lines at the amplitude levels chosen. Such signals are then misinterpreted as dial tone signals and the diverting device resets, thereby interrupting conversations. Certain types of noise or conversation may produce this abort result, as may Touch Tone (R) signals, background microwave noise where a long distance call is being carried on microwave networks, and tape hiss where the diverter reaches a telephone answering machine with a prerecorded tape message. Also, these detectors frequently fail when lines A and B are on different exchanges.
To avoid all of these problems, the improved dial tone detector of the present invention looks instead for the persistence over an appropriate time period of a signal in only that limited portion of the audio frequency range where dial tone signals are found. Thus, instead of seeking a constant amplitude over a frequency spectrum ranging from approximately 300 to 3000 Hz, the dial tone detector of the present invention looks for the persistence of an appropriate signal in the frequency range between approximately 180 and 800 Hz, which is where dial tone is found. The dial tone detection means of the present invention itself filters out signals below 180 Hz at the low frequency end of the dial tone spectrum and because an operational amplifier that feeds information from the telephone line to the dial tone detector contains a filtering device, the dial tone detector does not receive signals above 800 Hz at the high frequency end. A major object of the present invention is to accomplish detection of dial tone while at the same time avoiding the problem common in the prior art of misdetecting other signals commonly found on telephone lines as dial tone. In extensive testing, it has been found that the dial tone detection of the present invention will unfailingly detect any continuous tone in the appropriate frequency interval as dial tone and reject other non-dial tone signals. As a result, the call diverter claimed in the present application can be connected to two telephone lines from two different telephone exchanges or even from two entirely different telephone systems and still successfully forward calls, thereby overcoming a major problem in the prior art, namely, the common problem of calls being disconnected because a diverter is unable to recognize different types of signals from different exchanges or systems both as dial tones.
It is also an object of the present invention to provide such accurate detection without the necessity of any field calibration of the dial tone detector and at a significantly lower cost than has previously been possible even for less discriminating dial tone detectors.
Prior art devices have also failed to function well in a variety of telephone environments because of simple timing problems. As an example, the time it takes a telephone central exchange to furnish a dial tone to a subscriber varies widely from exchange to exchange, partially as a function of the equipment in the exchange and partially as a function of the volume of calls being handled at the time. Some prior art devices have initiated dialing solely as a function of time, and lost calls because of the failure to wait for a dial tone. Some prior art devices have bridged the incoming line to the outgoing line prior to establishing an interconnection on the outgoing line. Depending on when a particular prior art device bridged the lines, numerous problems were incurred. Sometimes line A was seized too early, causing a disconnect to the caller. Sometimes line A was seized during the dial out on line B, confusing the calling party. If line A was seized before determining that line B was available and a dial tone occurred on line B, the caller would hear the diverter operate and assume he had a bad line, thereby terminating his call. In any event, an early seizure of line A results in a completed call to the caller, with charges, even though the diverter malfunctioned or was unable, for any of a variety of reasons, to divert the call. The diverter of the present invention utilizes an elaborate sequence of electronic delays at crucial stages in the sequence of its operations which serve to compensate for varying operational conditions and varying speeds of the telephone company equipment, to compensate for the distances over which calls, incoming or outgoing, must travel, and for the psychological impact of a diverted call on the caller. While these delays are timed delays, the times are consistent with operational conditions most frequently encountered in telephone calling, both from the point of view of functioning equipment and from the point of view of the impatient caller. These timed delays also permit automatic bridging between lines A and B through an auto-bridging circuit which compensates for varying operational conditions.
While the dial tone detector, the timing sequence, the autobridging circuit, and the audio bridge represent the major improvements over prior art devices disclosed herein, other improvements are also present and will be disclosed in the description of the preferred embodiment. All such improvements are directed toward increased reliability, increased capability, improved compatability and lower manufacturing, installation and maintenance costs.